feat: 初始化核心数据库表结构、Go模型并包含日期字段类型迁移脚本

database/sql/migrations/20260128_transaction_date_to_datetime.sql

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+-- Migration: Change transaction_date from DATE to DATETIME
+-- This migration updates the transaction_date column to store both date and time.
+-- Run this SQL on your MySQL database.
+
+-- Step 1: Modify the column type (preserves existing data - dates will have 00:00:00 time)
+ALTER TABLE `transactions` MODIFY COLUMN `transaction_date` DATETIME NOT NULL;
+
+-- Step 2 (Optional): Migrate existing transaction_time data into transaction_date
+-- This updates existing records to combine date + time into the transaction_date column
+-- Only run this if you have existing data that you want to preserve time info for
+UPDATE `transactions` 
+SET `transaction_date` = TIMESTAMP(DATE(`transaction_date`), IFNULL(`transaction_time`, '00:00:00'))
+WHERE `transaction_time` IS NOT NULL;
+
+-- Note: After running this migration, the transaction_time column is no longer used
+-- but is kept for backward compatibility. New transactions will store full datetime
+-- in the transaction_date column.